Compared to thermoset polymers, thermoplastic polymers have several advantages, and the one most relevant to rapid forming is their ability to be reshaped [31]. Recyclability is another major advantage of thermoplastic over thermoset [24]. Polypropylene (PP) is a good choice for the matrix of natural fibre reinforcements due to its low cost, low
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processing temperature (to facilitate the low thermal stability of natural fibres), and strong hydrophobic character (which protects the hydrophilic natural fibres) [25].
Many attempts have been made to study the shortcomings of natural fibres, efforts directed to maximise their applications in a range of industries. In general, there are two major issues associated with natural fibre composites: low thermal resistance as well as moisture absorption. Due to their inherently low elongation-to-failure, natural fibre composites usually require additional treatments before forming into parts having a high level of complexity. In auto parts manufacturing, the poor thermal resistance of natural fibre composites is a critical barrier for widespread use because the conventional treatment, preheating, is often not applicable to this class of material system. Thermogravimetric analysis (TGA) has been frequently used to study the thermal stability of natural fibres. In a typical situation, the fibre suffers a slow weight drop when it is subjected to heat, followed by a sharp drop over a narrow range of temperatures and finally returns back to the stable state as reactants are exhausted [17]. Due to similar characteristics, natural fibres usually share a similar TGA curve in which almost 60% of the thermal decomposition takes place between 215˚C and 300˚C [32]. It has been reported that lignin starts degrading at lower temperatures compared to the other major constituents of natural fibres, cellulose and hemicellulose [33]. A better thermal resistance can therefore be anticipated in natural fibres with low lignin content, such as flax fibres [34], or in those natural fibres in which the lignin has been removed by chemical treatments [33]. The thermal degradation of natural fibres is therefore an important issue associated with manufacturing natural fibres reinforced composites, especially since preheating is traditionally used to improve the formability of composites during rapid forming.
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The variations in structure as well as properties of natural fibres result in variation in the performance of natural fibre reinforced composites when they are subjected to moisture absorption processes. Athijayamani et al. [35] studied the performance of a roselle/sisal fibre reinforced polyester hybrid composite when subjected to moisture ingression process, and the maximum weight gain reported was 4%. A significantly higher saturation level was observed by Wang et al. [36] for rice hulls/low density polyethylene composites which reached a stable weight gain of 17% after being immersed in distilled water for approximately 100 minutes.
Reduced mechanical properties of sisal/polypropylene composites have been observed in tensile tests as a result of increased water uptake, time of immersion, and fibre loading [37], and the driving mechanisms behind these variations was determined as the weakening of the fibre/matrix interface. Le Duigou et al. [38] investigated a flax fibre reinforced PLA composite with 20% fibre loading, and the composite, when saturated, exhibited an almost halved strength, and significantly reduced stiffness, but a doubled elongation-to-failure (compared to the untreated condition). This research also showed that variations in elongation-to-failure were temporary, while those in strength and stiffness were permanent. Through SEM examination of fractured surfaces, the changes in mechanical behaviour of the composite were explained by fibre plasticization as well as increased ductility of the PLA matrix. Similar observations have also been reported on a sisal fibre reinforced polypropylene composite [39].
Variations in mechanical properties are reported when natural fibres are exposed to different chemical solutions. Methacanon et al. [22] investigated variations in the tensile properties of water hyacinth, reed, roselle and sisal fibres caused by moisture absorption processes. A considerable increase in tensile strength was reported in all fibre yarns, and
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the study attributed these observations to a higher amount and better orientation of crystalline cellulose in fibres. Extra extensions and elongations were also found in wet yarns compared to dry equivalents as the absorbed water behaved like a lubricant so that one fibre could slide over another. Goda et al. [40] determined the effect of alkylation on the tensile properties of ramie fibres, where the fibres were alkali-treated with a 15% NaOH solution. A 4-18% improvement in tensile strength accompanied with a reduction in stiffness was noticed in the treated fibres. A significant increase in ductility was also observed so that the treated fibres exhibited a more than doubled elongation- to-failure. Such improvements in properties were considered to be a result of changes in the morphological and chemical structure of the fibres. These observations contradict those obtained by Ray et al. [41] in which an increased modulus and a reduced elongation-to-failure were noticed when jute fibres were subjected to an alkaline treatment (a 5% NaOH solution at 30˚C).
Cellulosic fibres are incompatible with hydrophobic polymers due to their hydrophilic nature, which usually results in poor interfacial adhesion between two materials and hence leads to voids within the composite. It is therefore not surprising that natural fibre polymeric composites are also sensitive to moisture, and the corresponding damage can be accelerated at high temperatures [17]. As stated in Table 2.1, hemicellulose is the fibre component which is responsible for moisture absorption. A higher moisture absorption level is therefore expected for those natural fibres that have a higher content of hemicellulose. Biological activities such as fungal growth can contribute to biodegradation of composites after they have been exposed to moisture for a long period of time [42]. The hydrophilic nature of natural fibres makes their polymeric composites more prone to absorb moisture compared to polymers. An increased rate of moisture absorption was reported when the volume fraction of natural fibres increases [17]. Apart
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from natural fibres, moisture exposure can also alter the interfacial conditions between different constituents of the composite. Under such circumstances, intermolecular hydrogen bonds are established between the water molecules and the cellulosic fibres, causing reduced adhesion between the fibre and the matrix [17]. A weakened fibre/matrix interface is also attributed to the fact that cellulosic fibres tend to swell when the composite absorbs solutions [43].
Moisture absorption of natural fibre composites enables the introduction of chemical treatments. Xu et al. [44] modified a kenaf fibre surface with a silane coupling agent to enhance adhesion between the fibres and the polystyrene matrix. A condensation reaction between alkoxysilane and the hydroxyl groups of the kenaf fibres took place, resulting in a higher storage modulus and hence an improved interaction between different constituents. A similar observation of an improved storage modulus of natural fibre composites after silane treatment was found in abaca fibre reinforced polyester composites [45].
Unlike other chemical treatments which only modify fibre surfaces, maleic acid (M.A) also modifies the polymer matrix, and lead to better bonding between the fibres and the matrix. Consequently, maleic acid can achieve better adhesion between two constituent materials compared to other treatments. Kiekens and Velde [46] treated dew-retted hackled long flax with propyltrimethoxysilane, phenylisocyanate and maleic acid anhydride modified polypropylene, and the highest increase in interfacial strength was observed in the maleic acid treated composite. This work also showed that when a flax/polypropylene composite is treated with maleic acid, chemical entanglements form within the maleic anhydride/polypropylene copolymer and that this physical link improves load transfer between the fibres.
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